Hearing assistance devices, such as hearing aids, amplify received sound to assist the hearing of the wearer. Modern devices tailor the amplification to attempt to restore natural hearing to the wearer of the device. In the case of hearing aids, a microphone receives sound, processes it to meet the needs of the wearer, and produces audible sound to the wearer's ear using a receiver, also known as a speaker. Some hearing aids are designed to occlude the ear canal, and thereby reduce the amount of sound transmitted back from the receiver to the microphone. In such devices, attenuation of sound reaching the microphone from the receiver is used to prevent feedback from becoming oscillation. This allows the hearing aid to use more amplification without ringing or squealing oscillations.
Some devices use a non-occluding approach, whereby amplified sound is provided to the ear canal, but in a way where an open passageway for sound is provided to the ear. Such designs must be careful with use of gain, since there is a higher probability that sound from the receiver will feed back into the microphone of the hearing aid as oscillations.
In both occluding and non-occluding devices, determination of the amount of amplification that can be used, or gain margin, before oscillating is difficult. One way this is done is to reduce gain of the device until oscillations disappear. Such an approach is crude and inefficient since gain margins vary over the sound hearing frequency ranges. Thus, if not done properly, the frequencies most likely to result in oscillation limit the available gain for the remainder of the hearing frequencies.
What is needed in the art is an improved system for determining the amount of available gain margin as a function of frequency. The system should be straightforward to implement in uses with hearing assistance devices.